well first of all use ' href="javascript::previous()" ' . Always use dobule quotes as this is proper coding.
Secondly, instead of using ' document.slideshow ' use ' document.getElementById('slideshow') ' and add to the img tag an id property like: ' <img src="tropic.jpg" name="slideshow" id="slideshow"> ' .
If these things does not help and still nothing changes I can suggest using something like this as javascript:

I just changed the changing src to an innerHTML method. Well this code should work if I'm not missing something but you could also try your previous code by changing what I said before.
Hope that helps